McCartney A, et al. - Among participants from a double-blind, double-dummy, randomized trial, the Evaluation of Faslodex versus Exemestane Clinical Trial (EFECT), researchers evaluated if serum thymidine kinase 1 (TK1) activity (sTKa) correlates with patient outcome. In EFECT, postmenopausal women with advanced breast cancer who had disease progression on non-steroidal aromatase inhibitor therapy were given fulvestrant or exemestane. Here, experts performed retrospective analyses of serum archived from EFECT. For patients with low baseline sTKa levels vs those with high sTKa baseline levels, 5.03 months vs 2.57 months, respectively, was the median time to progression. In patients with advanced breast cancer treated with endocrine therapy, sTKa was identified as a potential circulating prognostic marker, and could allow upfront detection of endocrine therapy resistance and early positive response to therapy. Patients whose sTKa increased from baseline with treatment had a significantly shorter median time to progression vs those without an sTKa increase.